1. Field of the Invention
The present invention relates to the field of chopper stabilized operational and instrumentation amplifiers.
2. Prior Art
Chopper stabilized operational and instrumentation amplifiers are well known in the prior art. In a typical operational amplifier, the signal amplification path includes a plurality of cascaded amplifiers, or stages of amplification defining the signal path. If the amplifier input were shorted, the input offset primarily of the first amplifier or stage would be amplified, typically to saturate the output of the operational amplifier. When used in a feedback circuit, this does not happen, but instead the input offset causes the input to the amplifier to effectively be equal to the actual input to the circuit shifted by an amount equal to the input offset. Ideally, with a very high DC gain in the signal path, the input to an operational amplifier circuit in a feedback application will be very near zero, substantially independent of the operational amplifier output.
The input offset of integrated circuit operational amplifiers is reasonably low and satisfactory for many applications, but not the higher precision applications. Using an operational amplifier as an example, in the prior art, in order to cancel at least part of the input offset, the input of a chopper amplifier is also coupled to the amplifier input, with the output of the chopper amplifier being integrated and the output of the integrator being combined with the signal in the signal path after at least some signal path amplification. Since the main contributor of offset in the signal path is the input or first amplifier in the cascaded amplifiers, injection of the offset correction after at least the first stage of the cascaded amplifiers substantially reduces the effective input offset of the cascaded amplifiers in the signal path. If the gain of the chopper amplifier path is high, it will even cancel the offset of cascaded stages following that input stage. The chopper amplifier (input and output choppers enclosing an amplifier) converts the input offset to AC by the input chopper and amplifies the AC, with the output chopper operating at the same frequency reconverting AC to DC responsive to the input offset in the signal path for integration and then injection into the signal path. The net effect is that any input offset of the cascaded amplifiers defining the signal path results in an input to the integrator, which integrates the input offset and injects a DC result into the signal path to drive and maintain the input offset of the cascaded amplifiers substantially at zero. This is a substantial improvement in input offset of higher precision operational amplifiers and instrumentation amplifiers.
However, there are various other sources of offset as well as sources of noise in such a configuration. Not only is offset undesirable because of its effect on accuracy, but also chopper induced noise is undesirable, and may cause problems especially in systems capable of responding to such frequencies or whose performance is degraded by noise in the system.